1. Field of the Invention
The present invention relates to a circuit and system for driving an organic thin-film electroluminescent (EL) display to emit light, and particularly to a circuit and system for driving the organic thin-film EL element to emit light at a specified constant driving current.
2. Description of Related Art
The light-emitting luminance of the organic thin-film EL elements varies when the driving current flowing into the element varies. To control the uniformity of luminance of organic thin-film EL element, the driving current flowing into the element must be controlled and maintained at a specified constant current level among the organic thin-film EL elements.
FIG. 1 shows a prior art driving circuit. In FIG. 1, a constant current supply 13 intends to change the driving current, which is supplied from a power supply 11 to a light-emitting element 12. It should be noted that the light-emitting element 12 emits light when a switch 14 is open as indicated by solid line, and ceases to emit light when the switch 14 is closed as indicated by dotted line.
FIG. 2 shows another prior art driving circuit. In this configuration, a high resistance 15, which is inserted in series between a light-emitting element 12 and a power supply 11, intends to control the driving current flowing through the light-emitting element 12 to be a constant. It should be noted that the light-emitting element 12 emits light when a switch 16 is located at a position indicated by solid line, and ceases to emit light when the switch 16 is changed to another position indicated by dotted line.
The organic thin-film EL element can be modeled as an equivalent circuit composing a diode 32 and a parasitic capacitor 31 connected in parallel, as shown in FIG. 3. The parasitic capacitor 31 within the equivalent circuit always causes a response problem, especially in a matrix of organic thin-film EL elements. The organic thin-film EL elements cannot emit light normally unless a voltage difference between both ends exceeds a specified forward voltage Vf. The forward voltage Vf of LED is as low as +1.5 V to +2 V and also relatively stable. On the other hand, the forward voltage of the organic thin-film EL is as high as +5 V to 12 V and also greatly vanes in accordance with luminance, temperature and time passage. Besides, the parasitic capacitance effect is more severe in an organic thin-film EL element than in a LED due to a higher forward voltage Vf. The forward voltage Vf has to rise above the specified voltage value for luminance and the rise time is depended on the total charging time of all the parasitic capacitors parasitizing in the organic thin-film EL elements. Normally, the power supply is required to boost to a Vcc voltage potential higher than the forward voltage Vf in order to drive the organic thin-film EL element to emit light.
FIG. 4 shows a prior art driving system 40 for driving luminous elements. In FIG. 4, the prior art driving system 40 is constructed with a matrix arrangement of the number of Nxc3x97M (only 6xc3x975 organic thin-film EL elements appear in FIG. 4), in which the cathode-scanning unit consists of N number of cathode scanning lines. The cathodes of organic thin-film EL elements are connected to the switches 71 to 7n through the cathode scanning line X1 to Xn for selecting a power potential VB or a ground potential. The anode data-driving unit consists of M number of anode data-driving lines. The anode data-driving lines Y1 to Ym are individually connected to the switches 111 to 11m with constant current supplies 101 to 10m and ground. The prior art driving system 40 causes the luminous elements at an arbitrary intersection to emit light by selecting and scanning one of the anode lines and the cathode lines sequentially at fixed time intervals.
Accordingly, the prior art driving system 40 always causes problems once used in driving a matrix of organic thin-film EL elements for luminance. The main problem is that the scanning speed will be slowed down due to the parasitic capacitors described above. When the organic thin-film EL is used as a luminous element, this problem becomes more severe since the organic thin-film EL has a large capacitor to generate a surface emission. The above problem is more severe when the number of the luminous elements increases since the organic thin-film EL will to accumulate all the parasitic capacitors. Furthermore, the parasitic capacitors of all luminous elements connected to the anode lines have to be charged, and the current sources for driving the luminous elements connected to each anode line must be designed large enough to satisfy the appropriated response time. This requirement for generating large current sources is detrimental from the aspect of miniaturization of the circuit.
FIG. 5 is a timing chart of the driving system shown in FIG. 4. FIG. 5 shows the parasitic capacitor problem in the switching operations of the switches 7ixe2x88x921, 7i+1, 7i+1, and 11j. The potential of Yj data electrodes cannot increase at once due to the parasitic capacitance in the reverse bias direction of at least (nxe2x88x921) pixels. A delay time td occurs until a forward bias is applied to the pixel D(i, j) for light emitting. In addition, the current source 10j will limit the increasing rate of the potential of the Yj data electrodes and results in a larger delay time td.
FIG. 6 shows a current response when an input voltage pulse is applied to an organic thin-film EL element. In FIG. 6, a curve 61 represents the organic thin-film EL element current response, and a curve 62 represents the voltage pulse. It is clear that the rise time is longer than the fall time. This indicates that the time for capacitance discharge is shorter than the time for capacitance charge in the organic thin-film EL element. The advantage of a shorter capacitance discharge time can be used to develop a fast response driving circuit for an organic thin-film EL display. In the prior art driving system shown in FIG. 4, a constant current source 10j is connected to a set of parallel organic thin-film EL elements, D(l, j) through D(n, j), following to the ground potential in D(i,j) and to reverse power potential in rest of D(l to ixe2x88x921, j) and D(i+1 to n, j). Normally, the constant current source is sourcing a magnitude of current to light up an organic thin-film EL element. It should be noted that the parasitic capacitors in parallel could enhance the parasitic capacitance effect compared to that of a single organic thin-film EL element. The current source limits the current and worsens the response to emit light of the scanned organic thin-film EL element D(i, j) due to the above parasitic capacitance effect when a power potential is applied. Several methods to improve the response to emit light in prior art organic thin-film EL display driving system is proposed in U.S. Pat. No. 6,201,520 and No. 5,844,368. However, the above methods do not really resolve the existent problems.
The object of the present invention is to resolve the problems and disadvantages of the related art. The present invention provides a driving circuit for driving an organic thin-film EL element to emit light. Furthermore, a driving system organized by the driving circuits of the present invention is applied to drive an organic thin-film display.
In a first embodiment of the present invention, a driving circuit for driving an organic thin-film EL element comprises an anode-scanning switch, an organic thin-film EL element, a constant current source and a cathode data-driving switch. The anode-scanning switch is connected to a power potential while being scanned and connected to a ground potential otherwise. The organic thin-film EL element is connected to the anode-scanning switch. The constant current source is connected to the organic thin-film EL element. One end of the cathode data-driving switch is connected to the constant current source, and another end of the cathode data-driving switch is connected to a ground potential while the organic thin-film EL element is selected. Otherwise, the other end of the cathode data-driving switch is connected to a power potential.
In a second embodiment of the present invention, a driving circuit for driving an organic thin-film EL element comprises an anode scanning unit, an mxc3x97n matrix of organic thin-film EL elements, n columns of constant current sources, a cathode data-driving unit and a signal control unit. The anode scanning unit includes m rows of anode-scanning switches, each anode-scanning switch connected to a power potential while being scanned and connected to a ground potential otherwise, wherein m is an integer. The organic thin-film EL elements at the same row are connected to a corresponding anode-scanning switch. The organic thin-film EL elements at the same column are connected to a corresponding constant current source. The cathode data-driving unit includes n columns of cathode data-driving switches, one end of each cathode data-driving switch connected to the constant current source, another end of the cathode data-driving switch connected to a ground potential while the organic thin-film EL element is selected and connected to a power potential otherwise. The signal control unit is used to switch the anode-scanning switches and the cathode data-driving switches.
In order to enhance the response to emit light of pixels composed by the organic thin-film EL elements in a line during the line scanning, the driving system for driving the organic thin-film EL display includes a plurality of intersecting anodes and cathode lines arranged in a matrix, a matrix of organic thin-film EL elements, a plurality of constant current sources and a signal control unit. In this driving system, the anode lines are scanning lines, and the cathode lines are data-driving lines corresponding to the driving circuit in the first embodiment of the present invention; each of the organic thin-film EL elements is coupled to one of the scan lines and one of the driving lines at a point where the scan lines and driving lines intersect. The scanning lines and driving lines are connected and controlled through the signal control unit. Each driving line is connected to a constant current source before connecting to the signal control unit, which can cause at least one of the organic thin-film EL elements to emit light by scanning one of the scan lines for a predetermined period of time in a scanning process and which is coupled to the data-driving lines. In order to increase the response to emit light in the organic thin-film EL display, the data pulses are set at least one clock time ahead of the scanning pulse. The signal control unit sets a power potential to a scan line by coupling the rest of the scan lines to ground potential.
By the construction described above, when the scanning position is switched to the next scan line with a power potential and the rest of the scan lines are set to a ground potential, the parasitic capacitor of the organic thin-film EL element which emits light is charged by the scanning source via the scan line, and the parasitic capacitor of the organic thin-film EL element that does not emit light is charged under the presence of the reverse bias voltage of the driving lines at the same time. The arrangement allows an instant build up of a forward voltage for the organic thin-film EL element that is to emit light, and the organic thin-film EL element can quickly respond to emit light.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.